https://leetcode.cn/problems/missing-number/submissions/541143773/
class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int ret=0;
        for(int i=1;i<=nums.size();i++)
        {
            ret^=nums[i-1];
            ret^=i;
        }
        return ret;
    }
};